Quick writeup for ARAM Soraka: designated Warmog's holder.

Warmog's requires 550 extra health for passive to activate.
Usually, you will have enough gold to buy Warmog's and extra HP around level 10.

• 200 HP from 2 scaling HP shards at level 10.
• 150 HP from 1 ruby crystal
• 200 HP from 1 kindlegem

That gives 550 HP.

Kindlegem then builds into moonstone, and ruby crystal into whatever.

Tear and Presence of Mind is mandatory, otherwise you will run into mana problems and become useless.
If you don't have a tanky frontline to help you get in AA range of enemy champions to proc Presence of Mind, take Clarity summoner spell.

Resolve Tree Build
If your team is strong early game, you might be able to rush the Warmog's and HP items before level 10. In this case, the Overgrowth rune can be taken to make up for the 20 HP deficit.

Otherwise, just take the Revitalize rune.

Domination Tree Build
Mostly worse than Resolve, except for Treasure Hunter which can help get Warmog's online earlier.
Predator is kinda jank but can help you catch up to teammates diving backline as long as you don't hit enemy champions.